a. Field of the Invention
This invention relates to a device for cleaning the interior surfaces of pipes, propelled therethrough by a pressure gradient therein.
B. Description of the Prior Art
In the operation of pipelines, it periodically becomes necessary to clean from the inner surfaces of the pipes accumulations and deposits of sludge, scale, debris, and other material. These cleaning operations are most commonly carried out by propelling through the pipe, by a pressure gradient therein, devices known in the art as pigs.
There are a variety of designs of pigs available, most of which consist generally of a mass of resilient material having a generally circular cross section with diameter slightly greater than that of the pipe through which they are to be propelled. These pigs are most commonly cylindrical in shape, such as those described in U.S. Pat. Nos. 3,543,323 and 3,277,508. Among other designs for pigs are spheres, described in U.S. Pat. No. 3,543,324, and devices with resilient material mounted upon a rigid central shaft, as described in U.S. Pat. Nos. 3,484,886 and 3,541,628. Cylindrical pigs generally have rounded or pointed forward ends, examples of which are described in U.S. Pat. Nos. 3,538,531 and 3,277,508. Having a rounded or pointed forward end enables the pig to pass obstructions more easily and to negotiate bends in the pipe. Additionally, some cylindrical pigs have a concave back end -- examples of which are described in U.S. Pat. Nos. 3,538,531 and 3,602,934 -- which transmits a radial component of the force thereon caused by fluid pressure to the walls of the pipe, effecting a tighter fit. Finally, most cylindrical pigs are covered with strips of material which is harder than the resilient material of which the pig body is made. Additionally, some strips are applied to the pig in a helical pattern, serving to impart spin to the pig, thereby equalizing the wear over the surface of the pig. Examples of various striping patterns are shown in U.S. Pat. Nos. 3,204,274, 3,605,159 and 3,389,417.
One system for introducing pigs into a pipeline is described in U.S. Pat. No. 3,266,076. A supply of pigs is inserted into a launching tube having essentially the same diameter as the pipeline and joined to the pipeline in such a way as to allow a smooth passage of the pig into the line. Spaced longitudinally along the launching tube is a series of ports, connected by valved conduits, to a high pressure fluid supply line. The pigs are inserted into the launching tube in such a manner that spaces are left between the pigs, such spaces coinciding with the port spacing. To launch a pig, the valve immediately upstream of the most downstream pig in the launching tube is opened, allowing high pressure fluid to flood the space between the most downstream pig and the next most downstream pig. The pressure of the fluid upon the upstream end of the pig to be launched, being greater than that upon the downstream end thereof, forces the pig to move downstream in the launching tube and into the pipeline.
Cylindrical pigs, as presently known in the art, are manifestly unsuited for use in the launching system described above. Since the operation of the system depends upon there being spaces between the pigs coinciding with the location of the ports, each pig must be rather precisely placed within the launching tube. To achieve such placement, personnel are required to ram each individual pig, independently of the other pigs, a specified distance into the launching tube.
In addition to the problem associated with achieving proper initial spacing, a more severe disadvantage lies in the fact that the injection of high pressure fluid into the launching tube accomplishes not only the desired result of forcing the most downstream pig into the pipeline, but also the undesired result of forcing the upstream pigs further upstream in the launching tube. The upstream pigs may be so dislodged in the launching tube as to occlude the ports, making the system inoperable.